Modify FastD2I to use static_cast instead of lrint(). Benchmarks show that it's much faster this way.

Review URL: http://codereview.chromium.org/1128010

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4223 ce2b1a6d-e550-0410-aec6-3dcde31c8c00